1. Field of the Invention
The present invention relates to a method and a system for suppressing the formation of scale defects during hot finish rolling by minimizing scale formation during rolling of a strip material.
2. Description of Related Art
When iron contacts a gas, such as oxygen or air, at a high temperature during rolling of a strip material, a film of the reaction product, i.e., scale, is formed on the surfaces of the strip material. Due to the formation of scale, the strip material may become susceptible to adverse influences, such as oxidation. Therefore, the scale should be removed. The customary practice for removing scale formed on a strip material has been to jet pressurized water at the surface of the strip material.
FIG. 4 is a schematic view of a scale removing device of a conventional hot finish rolling mill system. FIG. 5 is a graph showing the temperature of a rolled material and the thickness of scale during finish rolling by a conventional hot finish rolling mill system.
With a conventional hot finish rolling mill system, as shown in FIG. 4, a plurality of finishing mills, i.e., 1st to 7th finishing mills 101, 102, 103, 104, 105, 106 and 107, are provided in a row along the direction of transport of a rolled material S, downstream of a roughing mill (not shown) in the direction of transport. The finishing mills 101, 102, 103, 104, 105, 106 and 107 have a pair of (i.e., upper and lower) work rolls 201, 202, 203, 204, 205, 206 and 207, respectively. A finishing mill group 100 is constructed in this manner. On the entrance side of this finishing mill group 100, a scale breaker 301 is provided for removing scale formed on the rolled material S. The scale breaker 301 has jet nozzles 302 positioned above and below the rolled material S. These jet nozzles 302 direct jets of water at a high pressure of, e.g., 200 kgf/cm.sup.2, at the upper and lower surfaces of the rolled material S to remove the scale.
Thus, the rolled material S transported after rough rolling from a slab by a roughing mill is conveyed to the entrance side of the finishing mill group 100, where scale formed on the surface of the rolled material S is removed by the scale breaker 301 before finish rolling. In detail, water that is pressurized at, e.g., 200 kgf/cm.sup.2, is jetted through the upper and lower jet nozzles 302 at the upper and lower surfaces of the conveyed rolled material S to remove the adhering scale. The descaled rolled material S is carried to the finishing mill group 100 for rolling by the work rolls 201, 202, 203, 204, 205, 206 and 207 of the 1st to 7th finishing mills 101, 102, 103, 104, 105, 106 and 107, whereby it is sequentially finish rolled to predetermined thicknesses.
FIG. 5 is a graph showing the temperature of the rolled material S and the thickness of its scale during descaling and finish rolling of this material. In this graph, A represents the period of scale removal by the scale breaker 301, and B, C, D, E, F, G and H represent the times of finish rolling by the 1st to 7th finishing mills 101, 102, 103, 104, 105, 106 and 107, respectively. This graph shows that the surface temperature of the rolled material S falls rapidly during the scale removal period A, and also drops at the finish rolling times B, C, D, E, F, G and H. At the same time, the scale is removed or thinned.
With such a hot finishing mill system, the rolled material S should be transported at a high speed in order to raise the work efficiency. When the rolled material S is transported at a high speed, however, its front end collides with the outer peripheral surface of the work roll 201, 202, 203, 204, 205, 206 or 207 when its front end portion is engaged into the finishing mills 101, 102, 103, 104, 105, 106, 107. As a result, the work roll 201, 202, 203, 204, 205, 206 or 207 maybe deformedor damaged. With the hot finishing mill system, therefore, the rolled material S has to be carried at a low speed, with the result that the rolled material S takes a long time until its engagement into the work rolls 201, 202, 203, 204, 205, 206, 207, promoting the formation of scale. Consequently, the thickness of the scale on the rolled material S after rolling exceeds a limit of 5 .mu.m. During finish rolling, this scale is imprinted into the surface of the rolled material S, causing defects. This deteriorates the quality of the rolled material S markedly.